Because international emission standards are becoming ever more stringent, especially for internal combustion engines of motor vehicles, new fuel injectors with actuation elements or actuators which react very quickly and largely without any delay have been developed for use in internal combustion engines. These are used for example for injection of fuel in diesel or Otto engines. Such actuators include piezoelectric elements, such as piezo actuators. For internal combustion engines with a number of cylinders the actuators of a number of cylinders are grouped into banks and each bank is then activated directly by an activation unit (e.g. output stage).
With a 4-stroke internal combustion engine 720° are needed, i.e. two complete revolutions, to run though all cycles once. To exclude the possibility of an activation unit activating two or more actuators simultaneously, the 720° operating range of the internal combustion engine or crankshaft angular range is divided up for each cylinder into operating windows of equal size. The operating window of a four-cylinder and 1-bank system amounts to 180° per cylinder. The operating window of a cylinder does not begin until the operating window of the previous cylinder ends, which means that the operating windows of the individual cylinders do not overlap and only a single operating window is active. During this active operating window the associated cylinder may be activated in any given way by the activation unit. For motors with a larger number of cylinders (6, 8, 10) the operating window is reduced for a 1-bank system in accordance with the following formula:Number of banks*720°/number of cylinders
With a 6-cylinder internal combustion engine this would only be 120°.
Because of legal requirements operating windows of 240° or more are needed.
To enable this to be achieved, a number of activation units are needed. Each cylinder bank is given one activation unit in each case.
Thus for example, with a 6-cylinder internal combustion engine, the first, third and fifth cylinders with their actuators can be activated by a first activation unit and cylinders two, four and six by a second activation unit. The operating window of a 6-cylinder internal combustion engine with two banks is consequently 240°.
This represents the injection segment or operating window of the respective cylinder during which fuel can be injected.
It has also proved to be of significance for adherence to the future high demands within the framework of the emission limits that in an individual segment the fuel for each cylinder should be divided up into a number of injections (so-called multiple injections). Of particular importance here is the explicit control of the injection amounts and the injection times of the fuel into the respective cylinder of an internal combustion engine with internal mixture preparation. Pilot injections typically bring about a soft and even increase in the combustion pressure, which for example markedly reduces the classic knocking of a diesel engine. A main injection is used to generate thermal energy, whereby in specific operating areas with a divided main injection, nitrous gas emissions can be greatly reduced. Secondary fuel injections reduce the raw emissions and the particle emissions and make it easier to regenerate possible downstream particle filters. An individual injection process can thus be constructed from pilot, main and secondary injections which are generated during the segment.
A method is known from EP 1497544 for operating a fuel injection system for an internal combustion engine in which at least two piezoelectric injectors are assigned to a bank in each case. The invention monitors whether a time interval in which a first piezo injector is to be charged or discharged overlaps with a time interval in which a second piezo injector is to be charged or discharged. In this case the injections are assigned a priority in each case. In the event of an overlap the injection with the lower priority is then shifted or shortened.
With a multiple injection however the segments and injection processes of a number of cylinders overlap, so that the number of banks would have to be increased, since each bank can only ever activate one injection process. FIG. 2 shows the problem with a schematic diagram of the timing sequence of an activation of a bank with cylinders Cy10 and Cy12. An injector (not shown) of the first cylinder Cy10 is activated so that three part injections 3.0, 4.0, 5.0 are undertaken, which have predetermined temporal positions in relation to the top dead center point TDCO in the cylinder and predetermined durations (i.e. amount of fuel delivered). After the injections are completed for the first cylinder Cy10, the activation unit brings about the injections 3.2, 4.2, 5.2 for the third cylinder Cy12. The change in the cylinder supplied with injections is represented by an arrow 6. Only when the injections 3.2, 4.2, 5.2 have been processed for cylinder CY12 can there be a switch, as depicted by arrow 6, to activation of the injector or of a further cylinder. In the method of operation in accordance with the prior art it can be clearly seen in FIG. 2 that the injection is possible for a further cylinder after processing of the respective previous cylinder injections and thus during the entire segment.
In addition a few injection concepts occasionally also need a very large segment (i.e. operating window) e.g. in a diesel particle filter regeneration mode a pilot injection is expediently undertaken with a crankshaft angle of −80° and a late secondary injection at 270°. If the same number of banks is retained however, this means a large overlapping of the segments of the individual cylinders, so that now and again an injection can be demanded simultaneously for a number of cylinders.